1. Field of the Invention
This invention relates to an improved apparatus and method for measuring particle size distribution.
2. Description of Related Art
It is known to provide particle apparatus which have a light emitting means to irradiate a sample of particles with light and to provide detection means to measure the light reflected and/or diffracted by the particles in order to determine the particle size distribution of the particles. Such known systems provide a plurality of detectors, generally in a plane, around a sample measurement zone. Such a system is shown in WO 90/10215 wherein an arc of detectors is provided substantially perpendicular to a beam of light incident upon the sample of particles.
It will be appreciated that the term particle may mean any phase of a discontinuous material contained within a continuous phase of a supporting medium. Either phase may be gaseous, liquid or solid. The only physical limitation is that the particle must have a different refractive index to the medium and further, that the medium must be substantially transparent at any illuminating wavelength of light.
According to a first aspect of the invention there is provided a particle size distribution analysis apparatus comprising a sample measurement zone adapted to define a sample of particles, a light emitting means adapted to provide a source of light incident upon the sample measurement zone, and at least a first detection means adapted to measure light levels in the apparatus at particular scattering angles and output a signal to a computation means enabling the particle size distribution of particles contained within the sample to be determined, wherein the computation means is adapted, in use, to calculate a particle size distribution taking into account reflections of light scattered off the particles by the measurement zone.
An advantage of such an approach is that the apparatus is more accurate than prior art apparatus. Previously particle size distribution analysis apparatus has measured the scattering of light from particles to perform the size distribution calculations. Reflection of the scattered light have led to inaccuracies in the calculations which were previously thought to be immaterial. It has been realised that the inaccuracies are in fact material and should be allowed for.
Preferably a second detection means is provided and the computation means is adapted, in use, to modify the readings taken from the first detection means based upon the readings taken from the second detection means to take in to account reflections.
Providing a second detection means and using readings taken from the second detection means to modify the readings taken from the first is beneficial in that the computational load on the computational means is reduced. Possibly the computational load is reduced by a factor of two.
Preferably the computation means is adapted, in use, to modify the readings taken from the second detection means based upon the readings taken from the first detection means to take in to account reflections. This may also reduce the computational load and improve the accuracy of the system.
The measurement zone may comprise at least a pair of spaced windows adapted to contain the sample. The windows may be glass, plastics, or some other material transparent to the light emitted by the light emitting means. The windows may be substantially parallel to one another.
Such windows have been used in prior art apparatus and are known. However, the materials used for the windows reflect light incident upon them. Such reflections are unwanted and can be detected by the detection means which interprets the reflections as scattering from the particles in the sample which lead to inaccuracies in the particle size distributions calculated by the computation means. The present apparatus may be adapted to modify the readings taken from the first detection means with readings taken by the second detection means in order to account for the reflections from the windows.
The windows may be coated in an anti-reflective coating which is adapted, in use, to reduce unwanted reflections.
The first detection means may comprise a large angle detector which may be situated substantially at a large angle from the axis of the beam of light emitted from the light emitting means taking the direction of travel of the light as 0xc2x0. A large angle may be substantially in the range 90xc2x0 to 40xc2x0 may be substantially 70xc2x0 to 40xc2x0.
The second detection means may comprise a back scatter detector which may be situated substantially at an obtuse angle from the axis of the beam of light emitted from the light emitting means taking the direction of travel of the light as 0xc2x0. The obtuse angle may be substantially in the range 90xc2x0 to 180xc2x0, may be in the range of about 110xc2x0 to about 170xc2x0.
There may be provided a plurality of back scatter detectors (second detection means). In the preferred embodiment there may be provided two back scatter detectors. In one embodiment the second detection means may comprise a detector placed at substantially 120xc2x0 and may comprise a further detector placed at substantially 135xc2x0.
There may be provided a plurality of large angle detectors (first detection means) and in the preferred embodiment there may be provided two large angle detectors. In one embodiment the first detection means may comprise a detector placed at substantially 45xc2x0 and may comprise a further detector placed at substantially 60xc2x0.
Preferably there are the same number of first and second detection means.
The light emitting means may be a laser which emits a beam of light.
Preferably the angle at which the first detection means and the second detection means are inclined symmetrically relative to the measurement zone. That is if the first detection means in inclined at an angle xcex8 relative to a beam of light emitted from the light emitting means the second detection means may be inclined at 180xc2x0 xe2x88x92xcex8. Indeed, if there are a plurality of first detection means and the same number of second detection means each of the first and second detection means may be inclined symmetrically to the measurement zone. Such an arrangement is advantageous in that it simplifies subsequent calculations to allow for the multiple reflections.
Preferably the first and second detectors are of substantially the same construction. This is convenient and makes the calculation of the multiple reflections easier; allowance does not have to be made for differences in the detectors.
According to a second aspect of the invention there is provided a method of improving the accuracy of a particle size distribution calculation performed by illuminating a sample with light from a light emitting means and taking readings of the amount of light scattered by the sample comprising providing at least a first detection means and calculating a particle size distribution taking into account reflections of light scattered from the particles by the measurement zone.
An advantage of such a method is that it can lead to more accurate results than prior methods. The method may be thought of as adjusting the data provided by the detection means to take account of known reflection terms. The modification of the readings may be used to allow for unwanted reflections within the apparatus which could lead to inaccurate results.
Preferably a second detection means is provided and the readings taken from the first detection means are modified by readings taken from the second detection means. This is advantageous in that the computational load is reduced.
Preferably the reading taken from the second detection means is modified by readings taken by the first detection means. Again, this may improve the accuracy of the system.
Preferably the method comprises compensating a reading from a detection means detecting light scattered having a directional component toward the light emitting means with a readings from a detection means detecting scattered light having no directional component toward the light emitting means. This may be thought of as compensating readings taken for back scattered light with readings taken for forward scattered light.
The method may also comprise compensating a reading from a detection means detecting light having no directional component toward the light emitting means with a reading from a detection means detecting scattered light having a directional component toward the light emitting means. This may be thought of as compensating readings taken for forward scattered light with readings taken for back scattered light.
Preferably the two detection means are provided at substantially symmetric angles relative to the measurement zone. This has the advantage that the calculations necessary to modify the readings are simplified.
The first detection means may comprise a plurality of detectors. Likewise the second detection means may comprise a plurality of detectors. The method may comprise integrating the signals received by the first detection means to obtain a reading and further comprise integrating the signal received by the second detection means to obtain a reading.
The method may be thought of as reducing the likelihood of the occurrence of inaccurate particle size distribution predictions. The unwanted reflections within the apparatus may lead to the detection means detecting particle sizes which are not actually present in the sample.